In a fuel cell system of a fuel-cell vehicle, output of a fuel cell thereof is controlled based on instructions from a vehicle control system.
While the fuel cell system is in warm-up right after the start-up thereof, or due to the deterioration of the system with age, the output characteristics of the fuel cell thereof change to have a lower output voltage. In order to obtain the required output power from the fuel cell at the lower output voltage, an operation point (output current and output voltage) of the fuel cell is shifted to have the output current increased to thereby compensate for shortage of the output power. However, under heavy load conditions at a relatively high output current, the output voltage sharply drops with increasing output current, which further decreases the output power. The system controls to have the output current further increased in order to gain the output power. This brings about a further drop in the output voltage, and consequently in this vicious circle, the required output power cannot be obtained.
Japanese Patent Application Laid-open No. 2002-231295 discloses a power control system in which an output characteristic of the fuel cell is corrected based on actual output current and voltage thereof, and an operation point is adjusted based on the corrected output characteristic.